I have a 1990 x-cab 4wd truck with a v6 and 4:56 gears, what were the original tire size for this model?

My speedometer is off by app 12 mph...fast....I have 31X10.50x15 tires and 4:56 gears, accordind to the code in the door...is this the correct size tire for this truck?...what can I do to correct this problem?

I also have a 1990 4runner....with 4:10 gears...it has 235x15 which is what it came with....I had 235's on my truck, but, compared to my 4runner, the rpm's were too high and the speedo was off by app 20mph...so I installed the 31x10.50's...now the speedo is only off by app 12mph...